This invention relates to a wire dot printer which prints by activating an actuator coil to select specific wire ends from a matrix of wire ends and, more particularly, to a circuit for activating the actuator coil within the print head to cause the wire ends to move.
Conventional wire dot print heads are known in the art and generally include a case which also acts as a magnetic core. A plurality of actuator coils are circumferentially disposed about the case. Print levers upon which the print wires are mounted are positioned within the case so that they are attracted by the coils when the coils are energized. The wires mounted upon the levers are caused to impact against paper held about a platen when the print levers are attracted by the coils.
Such conventional wire dot print heads have been satisfactory. However, such print heads are activated by a drive circuit which outputs voltage pulses in accordance with the characters to be printed. Because each coil included in the circuit has reactance, electric power builds up within the coils. Therefore, as shown in FIG. 14, if a neighboring switching device is turned ON to activate a particular coil, an electric current I.sub.0 will again flow to the actuator coil due to the electromotive force induced by a change in the residual magnetic flux stored in a magnetic circuit in the print head even though a printing operation has been completed and the print wires are returning to a home position during the activation of the second print wire. Therefore, the previously activated print wire is returned to its home position with accumulated delays. If the build up is too great or the accumulation too great, the result is protruding wires making printing impossible to perform.
Driver circuits are known for activating a wire dot print head which attempt to prevent electric current from flowing through the actuator coils when a change in residual magnetic flux stored in the magnetic circuit induces electromotive force during the return action of the lever. One such circuit is known from Japanese Utility Model Laid Open Application No. 191,032/88 which includes a driver circuit for activating the print head of a wire dot printer in which each actuator coil is connected to first switching terminal at one coil terminal and a second independent switching device at the other coil terminal. The first switching device receives a first driving signal in synchronism with printing. The second switching device receives a second driving signal which is produced in accordance with data related to the character to be printed. The terminal of each actuator coil connected to the first switching device is grounded by a diode while the other terminal is connected with a driving DC power supply through a counter electromotive force-absorbing circuit.
In the circuit configuration described above, a first driving signal having a pulse width T.sub.1 is input to the first switching device in synchronism with a timing control signal. A second driving signal having a pulse width T.sub.2 which is longer than the first driving signal pulse width is input to the second switching device for a selected actuator coil. The driving DC power supply energizes the actuator coil by supplying electric current which serially flows through the first switching device, the actuator coil, and the second switching device. This causes the coil to attract a selected lever moving the wire towards the platen.
After a period T.sub.1, the first switching device is turned OFF to deenergize the coil. The resulting counter electromotive force maintains an electric current flowing through the second switching device, the ground, a diode, and the actuator coil, thus maintaining the coil in an energized state. After a period T.sub.2, the second switching device is turned OFF. The resulting counter electromotive force induces electric current to serially flow through ground, the diode, the actuator coil, a second diode, and the counter electromotive force-absorbing circuit. In this way, electric current is fed to the driving power supply. As a result, the counter electromotive force induced in the actuator coil drops below the sum of the voltage developed by the DC power supply and the zener voltage of the voltage-regulator diode. For this reason, if another actuator coil is energized, no deleterious electric current is produced. Hence, the wire can be quickly returned to its home position by a spring.
In this driver circuit, the deleterious current produced during the return of the print lever can be reduced to a quite small value. However, magnetic flux still remains in the headactuating circuit including the actuator coils. If the second switching device is turned ON before the first switching device, an electrical path consisting of the diode, the actuator coil, and the second switching device is formed. The result, as can be seen in FIG. 15, is that electric current I.sub.0 ' is generated in the coil before the wire is to be actuated. Where the print speed is low and wires are driven at longer intervals of time, this current I.sub.0 ' due to the electromotive force attrituted to the residual magnetic flux poses no serious problems, because it is quite small. However, presently printers have been required to print at higher speeds. To comply with this requirement, wires are designed to stop within shorter time intervals and a higher voltage is applied to each actuator coil to supply a larger electric current in a shorter time. Because the actuator coils stop in a shorter time, the next character must be printed before the residual magnetic flux created by the previous energization has had an opportunity to dissipate. It also follows that the time between the instant at which one wire returns to its home position and the instant at which the next wire is started to be activated is reduced.
To obtain the required large electric current, the first switching device is connected with the DC power supply which delivers a relatively high voltage, usually on the order of 35 V. The first switching device is a PNP transistor. The base of a PNP transistor cannot be directly driven by a TTL circuit that has a maximum tolerance of more than 5 V. Therefore, it is the common practice to connect an NPN transistor before the first switching device to convert the driving signal into a higher voltage. On the other hand, the emitter of the second switching device is connected to the ground of the DC power supply and so the second device can be turned on by applying a signal exceeding a certain level, normally 0.6 V, to the base. Consequently, the second device can be driven directly with the driving signal from the TTL circuit.
If the driving signal is applied to the first and second switching devices simultaneously, the first device is turned ON after a delay equal to the time taken to turn ON the NPN transistor inserted in the front stage to increase the driving signal level. Even though, the driving signal for both switching devices is simultaneously supplied in response to a timing signal, there exists a period in which only the second device is in a conducting state. This conduction is combined with the electromotive force induced in the actuator coil due to the residual magnetic flux produced by the previous printing operation to thereby give rise to circulating electric current. The actuator coil for the next lever driven is energized earlier than intended. Thus, a character is printed at an incorrect time. As a result, the distance traveled by each successively driven wire decreases, as shown by the dotted line in FIG. 15. In a worst case, a printing wire is caught by the ink ribbon and becomes damaged or some dots fail to be printed, which leads to a deterioration of the print quality.
Accordingly, there is a need for a print head activating circuit for a wire dot printer which overcomes the deficiencies of the prior art by reducing the residual deleterious current remaining within an actuator coil from a previous printing operation.